This application proposes to study the nutritional regulation of selenium- containing enzymes and proteins. Glutathione peroxidase (GPX) is the best characterized selenoenzyme present in animals, and thus this research will have a major focus on GPX. GPX activity levels decrease rapidly during dietary Se deficiency, and we have used anti-GPX antibodies to show that GPX protein levels also fall exponentially and coordinately during progressive Se deficiency. Using a cloned GPX cDNA prove in Northern blot analysis, we have recently shown that the level of Se in the diet regulates the level of GPX mRNA as well as GPX activity and protein, thus suggesting that Se status of animals may regulate Se-containing enzymes such as GPX at the transcriptional level of gene expression. This regulatory mechanism maybe one of the major reasons why the dietary Se requirements for most higher animals, usually determined using GPX as an indicator, are so similar. In the last decade it has become increasingly clear that a number of selenoproteins in addition to GPX are present in the tissues of higher animals, but the nature and function of these selenoproteins are largely unknown. As with other species that are dependent on trace dietary factors for full biological activity, these species should be further characterized. Because activity assays, antibodies or cDNA probes are not generally available for these uncharacterized selenoproteins, our previously developed SDS/PAGE procedure can be used to identify and to quantitate 75Se-labeling of a number of these selenoproteins. This will allow initial studies on these relatively uncharacterized selenoproteins. To further our understanding of Se metabolism, we intend to pursue the following specific objectives: 1) to determine the effect of Se status on the expression of GPX (mRNA, protein and enzyme activity levels) in eukaryotes, and to further characterize this process; 2) to purify and to further characterize one or more of the uncharacterized mammalian selenoproteins. The overall objective of this research is to better understand the nutritional regulation of selenoproteins and selenoenzymes such as GPX. This knowledge may lead to a better understanding of the nutritional biochemistry of Se and it may increase our understanding of selenium's full role in animal and human health.